Inkjet printhead having robust encapsulation of wirebonds

ABSTRACT

An electronic assembly includes a substrate having a die and PCB mounted thereon. Wirebonds interconnect bond pads of the die with contact pads of the PCB, each wirebond having a first end portion bonded to a respective bond pad, an opposite second end portion bonded to a respective contact pad and an intermediate section extending between the first and second end portions. A dam encapsulant encapsulates each of the first and second end portions, a first fill encapsulant contacts the substrate and the dam encapsulant; and a second fill encapsulant overlies the first fill encapsulant. The first fill encapsulant has a lower modulus of elasticity than the second fill encapsulant and the dam encapsulant.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of U.S. ProvisionalPatent Application No. 63/065,329, filed on Aug. 13, 2020, thedisclosure of which is incorporated herein by reference in its entiretyfor all purposes.

FIELD OF THE INVENTION

This invention relates to encapsulation of wirebonds, particularly ininkjet printheads having one more printhead dies connected to a PCB. Ithas been developed primarily to provide robust chemical and mechanicalprotection of wirebonds whilst minimizing thermo-mechanical stressesthat may lead to issues, such as die-cracking.

BACKGROUND OF THE INVENTION

The Applicant has previously described methods for encapsulation ofwirebonds in inkjet printheads. For example, as described in U.S. Pat.No. 8,063,318 (the contents of which are incorporated herein byreference) wirebonds connecting contact pads on a printhead chip (or“printhead die”) and a PCB may be encapsulated using a combination of‘dam’ encapsulant beads covering contacts pads at opposite ends of thewirebonds and a ‘fill’ encapsulant, which encapsulates wirebondsextending therebetween. As described in U.S. Pat. No. 8,063,318, the damencapsulant has a relatively higher modulus of elasticity than the fillencapsulant, which provides sufficient control of mechanical stresses toavoid wirebond damage during thermal expansion/contraction of theprinthead.

U.S. Pat. No. 10,442,200 (the contents of which are incorporated hereinby reference) describes a printhead having a plurality of printhead diesattached to a metal alloy manifold via a metal alloy (e.g. Invar) shimSuch printheads have been designed for use with pigment-based inks and,further, enable construction of relatively long printheads, such as A3pagewide printheads.

As foreshadowed above, the primary function of a wirebond encapsulant inan inkjet printhead is to protect wirebonds from ink. If the encapsulantis breached and ink contacts the wirebonds, then the printhead will faildue to electrical shorting. However, it is equally important that theencapsulant itself does not introduce thermo-mechanical stresses in theprinthead that can lead to wirebond fracture or die-cracking, eitherduring manufacturing or during normal use via thermal cycling.

Relatively harder encapsulant materials—that is, encapsulant materialshaving a relatively high modulus of elasticity—are generally preferablein terms of mechanical and chemical robustness, particularly towardsaggressive cosolvents and surfactants found in certain inkjet inks. Forpigment-based inks used with the printheads described in U.S. Pat. No.10,442,200, relatively harder encapsulant materials have been found tobe necessary to avoid chemical attack. On the other hand, thoserelatively harder materials have a greater tendency to introduceundesirable thermo-mechanical stresses in the printhead. In particular,the printheads described in U.S. Pat. No. 10,442,200 are quitesusceptible to die-cracking during manufacture when a relatively harder‘fill’ encapsulant is used. This is understood to be a result of themechanical linkage provided by the encapsulant material between the diesand the PCB.

It would therefore be desirable to provide a wirebond encapsulant, whichis resistant to chemical attack whilst minimizing thermo-mechanicalstresses in the printhead, which may cause die-cracking.

SUMMARY OF THE INVENTION

An electronic assembly comprising:

a substrate;

one or more dies mounted on the substrate, each die having a pluralityof bond pads;

a PCB mounted on the substrate, the PCB having a plurality of contactpads;

a plurality of wirebonds interconnecting the bond pads and the contactpads, each wirebond having a first end portion bonded to a respectivebond pad, an opposite second end portion bonded to a respective contactpad and an intermediate section extending between the first and secondend portions;

a dam encapsulant encapsulating each of the first and second endportions, the bond pads and the contact pads;

a first fill encapsulant disposed on the substrate so as to contact atleast the substrate and the dam encapsulant; and

a second fill encapsulant disposed on the first encapsulant so as tocontact at least the first fill encapsulant and the dam encapsulant;

wherein:

the second fill encapsulant does not contact the substrate;

at least one of the first and second fill encapsulants encapsulates theintermediate sections of the wirebonds; and

the first fill encapsulant has a lower modulus of elasticity than thesecond fill encapsulant and the dam encapsulant.

The electronic assembly according to the first aspect advantageouslyprovides robust protection of the wirebonds whilst minimizingthermo-mechanical stresses that potentially lead to die-cracking orwirebond fracture. In particular, a relatively harder second fillencapsulant provides a mechanically robust and chemically-resistantouter layer while a relatively softer first fill encapsulant weakens themechanical linkage between the dies and the PCB, such that die-crackingeither during manufacture or during normal use is minimized. While thedies and the PCB are not fully mechanically decoupled, the relativelysofter first fill encapsulant is sufficient to minimizethermo-mechanical stresses to the extent that die-cracking becomesnon-problematic.

Preferably, the electronic assembly is a printhead and the dies areprinthead chips, such as MEMS printhead chips.

Preferably, the substrate is an ink manifold for delivering ink to theprinthead chips. The ink manifold may be comprised of, for example, apolymer such as liquid crystal polymer or a metal, such as Invar.

Preferably, the printhead chips are mounted on the substrate via anintervening shim. The shim may, for example, take the form of adouble-sided adhesive tape having opposite layers of adhesive disposedon a polymer support, as described in U.S. Pat. No. 7,347,534, thecontents of which are incorporated herein by reference. Alternatively,the shim may take the form of, for example, a film (e.g. a metal alloyfilm) adhesively bonded to the substrate, as described in U.S. Pat. No.10,442,200. Typically, the shim has ink through-holes defined thereinfor delivering ink from the ink manifold to the printhead chips.

Typically, the PCB is mounted directly on the substrate, for example,via adhesive bonding. The substrate may have a stepped mounting surfacefor accommodating the PCB.

Preferably, the dam encapsulant is configured as perimeter wall having apair of opposite longer walls covering the contacts pads and the bondpads, respectively, and a pair of shorter walls interconnecting thelonger walls at each end thereof.

Preferably, the first and second fill encapsulants are disposed withinthe perimeter wall, such that the dam encapsulant dams a flow of thefirst and second fill encapsulants during liquid deposition thereof.

Preferably, the second fill encapsulant is relatively more resistant tochemical attack than the first fill encapsulant.

Preferably, the first fill encapsulant has a modulus of elasticity inthe range of 20 to 200 MPa, or preferably 50 to 150 MPa.

Preferably, the second fill encapsulant has a modulus of elasticity inthe range of 500 to 3000 MPa, or preferably 700 to 2000 MPa.

Preferably, the dam encapsulant has a modulus of elasticity in the rangeof 500 to 3000 MPa, or preferably 700 to 2000 MPa. The second fillencapsulant and the dam encapsulant may be comprised of a same ordifferent materials.

Typically, the dam encapsulant, the first fill encapsulant and thesecond fill encapsulant are each comprised of epoxy resins. Epoxy resinencapsulants having different moduli of elasticity are commerciallyavailable from various suppliers (e.g. ResinLab, Chase Corporation,Engineering Materials Systems, Inc. etc.) and will be well known to theperson skilled in the art.

In a related aspect, there is provided a method of encapsulatingwirebonds in an electronic assembly, the electronic assembly having:

a substrate;

one or more dies mounted on the substrate, each die having a pluralityof bond pads;

a PCB mounted on the substrate, the PCB having a plurality of contactpads; and

a plurality of wirebonds interconnecting the bond pads and the contactpads, each wirebond having a first end portion bonded to a respectivebond pad, an opposite second end portion bonded to a respective contactpad and an intermediate section extending between the first and secondend portions,

wherein said method comprises the steps of:

depositing a dam encapsulant over each of the first and second endportions, the bond pads and the contact pads;

curing the dam encapsulant

depositing a first fill encapsulant so as to contact at least thesubstrate and the dam encapsulant;

curing the first fill encapsulant;

depositing a second fill encapsulant over the first fill encapsulant soas to contact the first fill encapsulant and the dam encapsulant; and

curing the second fill encapsulant

wherein:

the second fill encapsulant does not contact the substrate;

at least one of the first and second fill encapsulants encapsulates theintermediate sections of the wirebonds; and

the first fill encapsulant has a lower modulus of elasticity than thesecond fill encapsulant and the dam encapsulant.

As used herein, the term “modulus of elasticity” refers to the modulusof elasticity of an encapsulant material at 25 degrees centigrade afterbeing fully cured.

As used herein, the term “PCB” is taken to mean a printed circuit boardof the type having a non-conductive substrate and one or more conductivetracks carrying electrical signals. The non-conductive substrate may beflexible or rigid. The PCB may comprise additional electronic components(e.g. capacitors, resistors etc.) or, alternatively, the PCB may beabsent any additional electronic components and serve only to carryelectrical signals via its conductive tracks.

As used herein, the term “ink” is taken to mean any printing fluid,which may be printed from an inkjet printhead. The ink may or may notcontain a colorant. Accordingly, the term “ink” may include conventionaldye-based or pigment-based inks, infrared inks, fixatives (e.g.pre-coats and finishers), 3D printing fluids (e.g. binder fluids),biological fluids, functional fluids (e.g. sensor inks, solar inks etc.)and the like. Where reference is made to fluids or printing fluids, thisis not intended to limit the meaning of “ink” herein.

As used herein, the term “mounted” includes both direct mounting andindirect mounting via an intervening part.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way ofexample only with reference to the accompanying drawings, in which:

FIG. 1 is a front perspective view of an inkjet printhead;

FIG. 2 is a bottom perspective of the printhead;

FIG. 3 is an exploded perspective of the printhead;

FIG. 4 is a magnified cross-sectional perspective of the part of theprinthead;

FIG. 5 is a bottom perspective of part of the printhead;

FIG. 6 is a magnified bottom perspective of the printhead with a shieldplate and encapsulant removed for one row of printhead chips; and

FIG. 7 is a schematic side sectional view of a connection region betweena printhead chip and a PCB.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to an electronic assembly in its mostgeneral form. In one preferred embodiment, the electronic assembly takesthe form of an inkjet printhead, which is described in detailhereinbelow.

Referring to FIGS. 1 to 3 , there is shown an inkjet printhead 1, asdescribed in U.S. Pat. No. 10,442,200, the contents of which areincorporated herein by reference. The printhead 1 comprises an elongatemolded plastics casing 3 with ink connectors at each end thereof. Inletconnectors 7A of a multi-channel inlet coupling 8A protrude upwardsthrough openings at one end of the casing 3; and outlet connectors 7B ofa multichannel outlet coupling 8B protrude upwards through an opening atan opposite end of the casing (only two inlet connectors and two outletconnectors shown in FIG. 1 ). The inlet and outlet connectors 7A and 7Bare configured for coupling with complementary fluid couplings (notshown) supplying ink to and from the printhead.

The casing 3 has a first part 3A and a second part 3B positioned ateither side of a central locator 4, the first and second casing parts 3Aand 3B being biased towards each other and the central locator 4 bymeans of a spring clip 6 engaged therebetween. The two-part casing 3 incombination with the spring clip 6 enables the casing to expandlongitudinally, at least to some extent, to accommodate a degree oflongitudinal expansion in a main body 17 of the printhead 1.

The printhead 1 receives power and data signals via opposite rows ofelectrical contacts 13, which extend along respective sidewalls of theprinthead. The electrical contacts 13 are configured to receive powerand data signals from complementary contacts of a printer (not shown) orprint module and deliver the power and data to printhead chips 70 viarespective PCBs 18, as will be explained in more detail below.

As shown in FIG. 2 , the printhead 1 comprises a first row 14 and asecond row 16 of printhead chips 70 for printing onto print media (notshown) passing beneath the printhead. Each row of printhead chips isconfigured for printing two colors of ink, such that the printhead 1 isa full color pagewide printhead capable of printing four ink colors(CMYK) redundantly. The printhead 1 is generally symmetrical about alongitudinal plane bisecting the first row 14 and the second row 16 ofprinthead chips, notwithstanding the different ink colors in theprinthead during use.

In the exploded perspective shown in FIG. 3 , it can be seen that a mainbody 17 forms a rigid core of the printhead 1 for mounting various othercomponents. In particular, the casing 3 is snap-fitted to an upper partof the main body 17; the inlet and outlet couplings 8A and 8B(enshrouded by the casing 3) are connected to opposite ends of the mainbody; a pair of PCBs 18 are attached to a lower part of the main body(which are in turn covered by a shield plate 20); and a plurality ofleads 22 (which define the electrical contacts 13) are mounted toopposite sidewalls of the main body.

The main body 17 is a two-part machined structure comprising an elongateink manifold 25 and a complementary cover plate 27. The ink manifold 25functions as a carrier substrate having a unitary lower surface formounting the first and second rows 14 and 16 of printhead chips 70 aswell as respective PCBs 18. The manifold 25 and cover plate 27 areformed of a metal alloy material (e.g. Invar) having relatively highstiffness and a relatively low coefficient of thermal expansion. Incombination, the manifold 25 and cover plate 27 provide a stiff, rigidstructure at the core of the printhead 1 with minimal expansion alongits longitudinal axis. As foreshadowed above, the casing 3 is configuredso as not to constrain any longitudinal expansion of the main body 17and thereby minimizes bowing of the printhead during use. Accordingly,the printhead 1 may be provided as an A4-length printhead or anA3-length printhead.

Referring to FIG. 4 , an Invar shim 66 is adhesively bonded to a lowersurface 52 of the manifold 25 and a plurality of printhead chips 70,arranged in the first and second rows 14 and 16, are adhesively bondedto the shim 66 (only the first row 14 of printhead chips visible in FIG.4 ). Each row of printhead chips 70 receives ink from longitudinal inksupply channels 40 defined in the manifold 25 via through-holes in theshim 66.

A pair of longitudinal PCBs 18 flank the first row 14 and second row 16of printhead chips 70 at opposite sides thereof, each PCB being bondedto the lower surface 52 of the manifold 25. Each PCB 18 comprises arigid substrate (e.g. FR-4 substrate) for mounting of variouselectronics components and has one edge butting against a step 74defined in the lower surface 52 of the manifold 25. Each PCB 18 extendslaterally outwards beyond sidewalls 41 of the manifold 25. A shieldplate 20 is bonded to a lower surface of each PCB 18 and surrounds thefirst and second rows 14 and 16 of printhead chips 70 as well as acentral longitudinal region between the first and second rows. Theprotruding portions of each PCB 18 and the shield plate 20 defineopposite wings 75 of the printhead 1, while a uniformly planar lowersurface of the shield plate 20 is configured for engagement with aperimeter capper (not shown) surrounding both rows of printhead chips.

Still referring to FIG. 4 , a row of connection pads 80 extendslongitudinally along a distal edge portion of an upper surface of eachPCB 18. Each lead 22 has one end connected to a connection pad 80 andextends upwardly towards a respective sidewall of the main body 17. Theleads 22 have an upper portion mounted to a respective flange 29 of thecover plate 27 via a lead retainer 24 affixed thereto, and a lowerportion which flares laterally outwards towards the connection pads 80.Each lead 22 also has a portion defining the electrical contact 13 forconnection to external power and data connectors of a printer

Referring now to FIGS. 5 and 6 , an edge of each PCB 18 proximal arespective row of printhead chips 70 has a respective row of pinouts inthe form of contact pads 77. Each contact pad 77 is connected to arespective bond pad 73 on one of the printhead chips via a wirebondconnection (not visible in FIGS. 5 and 6 ). In this way, each row ofprinthead chips 70 receives power and data from the electricals contacts13 via the leads 22 and a respective PCB 18 adjacent the row ofprinthead chips.

The wirebonds are protected by an encapsulant package 79, which extendsbetween the proximal edge of each PCB 18 containing the contact pads 77and an proximal edge of the printhead chips 70 containing the bond pads73. As foreshadowed above, it is essential that the encapsulant package79 provides robust protection of the wirebonds, particularly withrespect to chemical attack from high pH inks, which typically containaggressive cosolvents and surfactants.

FIG. 7 shows schematically a side sectional view of a connection regionof the PCB 18 and the printhead chip 70, in accordance with the presentinvention. The printhead chip 70, having a longitudinal row of bond pads73, is mounted on a lower surface 52 of the ink manifold 25 via theintervening shim 66. The PCB 18 having contact pads 77 is directlymounted on the lower surface 52 of the ink manifold 25 adjacent theprinthead chip 70 and received in the stepped portion 74 of the lowersurface. A wirebond 90 interconnects the bond pad 73 and the contact pad77. The wirebond 90 has a first end portion 91 bonded to the bond pad73, an opposite second end portion 92 bonded to the contact pad 77 andan intermediate section 93 extending between the first and second endportions.

The encapsulant package 79 protects the wirebond 90, as well as thebonds pads 73 and contact pads 77, and comprises three components: (1) abead of dam encapsulant 95 extending longitudinally along the row ofbond pads 73 and the row of contact pads 77, which forms an endlessperimeter dam via transverse interconnecting portions at eachlongitudinal end thereof; (2) a first fill encapsulant 96 having arelatively low modulus of elasticity disposed within the perimeter ofdam encapsulant 95 on the lower surface the ink manifold 25 and anexposed portion of the shim 66; and (3) a second fill encapsulant 97having a relatively higher modulus of elasticity disposed on the firstfill encapsulant 96 within the perimeter of dam encapsulant. The damencapsulant 95 encapsulates the first and second end portions 91 and 92of the wirebond 90 as well as the bond pads 73 and contact pads 77,while the first and second fill encapsulants 96 and 97 togetherencapsulate the intermediate section 93 of the wirebond.

Importantly, the second fill encapsulant 97 does not contact the inkmanifold 25, which serves as a common supporting substrate for theprinthead chips 70 and the PCB 18. This has the effect of reducing themechanical linkage between the printhead chip 70 and the PCB 18 via theencapsulant package 79. Since only the first fill encapsulant 96, havinga relatively lower modulus of elasticity, is in contact with the inkmanifold 25 then any mechanical stresses invoked via thermal expansionof the ink manifold are minimized. On the other hand, the relativelyharder second fill encapsulant 97 provides a robust outer surface, whichis resistant to chemical attack as well as providing a mechanicallyrobust protective layer. Therefore, the encapsulant package 79significantly improves die packaging in harsh environments subject tothermo-mechanical stresses and chemical attack, such as those found ininkjet printheads.

In practice, it has been found that the intermediate section 93 of thewirebond 90 may be encapsulated within the first fill encapsulant 96,the second fill encapsulant 97 or, as shown in FIG. 7 , both the firstand second fill encapsulants. With the relatively softer first fillencapsulant 96, thermo-mechanical stresses in the connection region areminimized, and fracturing of wirebonds is minimized regardless ofwhether of the first or second fill encapsulant encapsulates thewirebonds 90.

An exemplary method for manufacturing an electronic assembly inaccordance with the present invention comprises the steps of: (1)forming wirebond connections between the bond pads of the die (e.g.printhead chip 70) and contact pads of the PCB (e.g. PCB 18); (2)dispensing a bead of dam encapsulant over the bond pads and contact padsand forming a perimeter wall; (3) curing the dam encapsulant using UVand/or thermal curing; (4) dispensing the first fill encapsulant andflowing within the perimeter wall of dam encapsulant; (5) curing thefirst fill encapsulant using UV and/or thermal curing; (6) dispensingthe second fill encapsulant and flowing within the perimeter wall of damencapsulant; and (7) curing the second fill encapsulant using UV and/orthermal curing.

It will, of course, be appreciated that the present invention has beendescribed by way of example only and that modifications of detail may bemade within the scope of the invention, which is defined in theaccompanying claims.

The invention claimed is:
 1. An electronic assembly comprising: asubstrate; one or more dies mounted on the substrate, each die having aplurality of bond pads; a PCB mounted on the substrate, the PCB having aplurality of contact pads; a plurality of wirebonds interconnecting thebond pads and the contact pads, each wirebond having a first end portionbonded to a respective bond pad, an opposite second end portion bondedto a respective contact pad and an intermediate section extendingbetween the first and second end portions; a dam encapsulantencapsulating each of the first and second end portions, the bond padsand the contact pads; a first fill encapsulant disposed on the substrateso as to contact at least the substrate and the dam encapsulant; and asecond fill encapsulant disposed on the first encapsulant so as tocontact at least the first fill encapsulant and the dam encapsulant;wherein: the second fill encapsulant does not contact the substrate; atleast one of the first and second fill encapsulants encapsulates theintermediate sections of the wirebonds; and the first fill encapsulanthas a lower modulus of elasticity than the second fill encapsulant andthe dam encapsulant.
 2. The electronic assembly of claim 1, wherein thedies are printhead chips and the electronic assembly is a printhead. 3.The electronic assembly of claim 2, wherein the substrate is an inkmanifold for delivering ink to the printhead chips.
 4. The electronicassembly of claim 3, wherein the printhead chips are mounted on thesubstrate via an intervening shim.
 5. The electronic assembly of claim4, wherein the shim comprises a metal alloy film.
 6. The electronicassembly of claim 4, wherein the PCB is mounted directly on thesubstrate.
 7. The electronic assembly of claim 1, wherein the damencapsulant is configured as an endless perimeter wall having a pair ofopposite longer walls covering the contacts pads and the bond pads,respectively, and a pair of shorter walls interconnecting the longerwalls at each end thereof.
 8. The electronic assembly of claim 7,wherein the first and second fill encapsulants are disposed within theperimeter wall, such that the dam encapsulant dams a flow of the firstand second fill encapsulants during liquid deposition thereof.
 9. Theelectronic assembly of claim 8, wherein the second fill encapsulant isrelatively more resistant to chemical attack than the first fillencapsulant.
 10. The electronic assembly of claim 1, wherein the firstfill encapsulant has a modulus of elasticity in the range of 20 to 200MPa.
 11. The electronic assembly of claim 1, wherein the second fillencapsulant has a modulus of elasticity in the range of 500 to 3000 MPa.12. The electronic assembly of claim 1, wherein the dam encapsulant hasa modulus of elasticity in the range of 500 to 3000 MPa.
 13. Theelectronic assembly of claim 1, wherein the dam encapsulant, the firstfill encapsulant and the second fill encapsulant are each comprised of amaterial selected from the group consisting of epoxy resins.
 14. Amethod of encapsulating wirebonds in an electronic assembly, theelectronic assembly having: a substrate; one or more dies mounted on thesubstrate, each die having a plurality of bond pads; a PCB mounted onthe substrate, the PCB having a plurality of contact pads; and aplurality of wirebonds interconnecting the bond pads and the contactpads, each wirebond having a first end portion bonded to a respectivebond pad, an opposite second end portion bonded to a respective contactpad and an intermediate section extending between the first and secondend portions, wherein said method comprises the steps of: depositing adam encapsulant over each of the first and second end portions, the bondpads and the contact pads; curing the dam encapsulant depositing a firstfill encapsulant so as to contact at least the substrate and the damencapsulant; curing the first fill encapsulant; depositing a second fillencapsulant over the first fill encapsulant so as to contact the firstfill encapsulant and the dam encapsulant; and curing the second fillencapsulant wherein: the second fill encapsulant does not contact thesubstrate; at least one of the first and second fill encapsulantsencapsulates the intermediate sections of the wirebonds; and the firstfill encapsulant has a lower modulus of elasticity than the second fillencapsulant and the dam encapsulant.
 15. The method of claim 14, whereineach curing step is selected from the group consisting of: thermalcuring and UV curing.
 16. The method of claim 14, wherein the damencapsulant is deposited as an endless perimeter wall having a pair ofopposite longer walls covering the contacts pads and the bond pads,respectively, and a pair of shorter walls interconnecting the longerwalls at each end thereof.
 17. The method of claim 16, wherein the firstand second fill encapsulants are disposed within the perimeter wall, andwherein the dam encapsulant dams a flow of the first and second fillencapsulants during respective depositions thereof.